Electrodeposition of zinc



Patented May 18, 1937 2,989,483 EmcTaoDEro'smoN F ziNc Richard 0. Hull, Lakewood, Ohio, assignor, by

mesne assignments, to E. Nemours & Company, Wilmington,

r tation of Delaware 1. du Pont do DeL, acor- No Drawing. Application April 4, 1935, Serial 12 Claims. (01. 204-18) f i with. While the use of addition agents improved This invention relates to the electrodeposition of zinc and is particularly directed to processes wherein a bright, mirror-like zinc deposit is plated from cyanide-zinc baths which contain substantial amounts of a thio'urea, and which. in addition, may also contain a metal of group VIII series 4 of Mendelejefis Periodic System.

The electrodeposition of zinc,'or electrogalvanizing, has been rather extensively employed, because electrodeposited 'zinc coatings, in addition to their low cost,'display many characteristics which cause them to be particularly desirable as protective finishes. W'hilezinc is not itself very resistant to corrosion, itdoes not accelerate the corrosion of iron or steel as. do such metals as copper, nickel, and chromium.

0n the contrary, zinc, being higher in the electromotive series, will protect iron or steel against rust even after appreciable. areas of the base metal are exposed. Despite their numerous advantages overrnany commonly used coating materials, electrodeposited zinc coatings have not enjoyed the use they deserve because they do not possess and do not retain a pleasing appearance and, consequently, for many purposes they are not acceptable.

The known methods of electrodepositing zinc almost invariably result in dark colored or dull plates, and, even when the deposits at first are fairly satisfactory, they soon become dark and discolored. The poor appearance of electrodeposited zinc coatings has limited their use almost exclusively to purely protective applications, and those working in the art have turned to other protective materials when it was desired to produce a finish of pleasing appearance.

, The electrodeposition of zinc has ordinarily been accomplished by the use of either an acidzinc bath ora cyanide-zinc bath. With neither of these baths has it been possible to obtain satisfactorily smooth and bright deposits, butthe acid-zinc bath is most commonly used because it leads to a brighter deposit with a better color than does the cyanide-zinc bath.

While, under favorable conditions, the deposits l obtained from acid-zinc. baths are relatively white, the deposits are still none too satisfactory because of their relatively coarse crystalline able by their use.

the character of the deposits to some extent, the results were'still none too satisfactory.

In addition to the fact, that acid-zinc baths do not produce satisfactory deposits, there are numerous other disadvantages attendant upon their use. Forone thing, acid-zinc baths have very poor throwing power, and it is exceedingly difilcult satisfactorily to plate irregularly shaped objects. Another disadvantage of acid-zine baths is their low cathode efllciency. As zinc is above hydrogen in the electromotive force series of metals, it is theoretically impossible to deposit zinc from acid solutions, but, of course, the rather great overvoltage of hydrogen does permit some zinc deposition. Concurrently with the deposition of zinc, however, there is a very considerable evolution of hydrogen. l a

While the deposits obtained from cyanide-zinc baths are poor in appearance, they have a relatively fine crystalline structure. A few addition agents, such as alum, gum arabic, and fluorides, have been tried in cyanide-zinc baths, but the results obtained were none too satisfactory. Aside from thepoor appearance of deposits obtainable therefrom,,cyanide-zinc bathshave a number of advantageous characteristics. They advantageous because the electric current applied to the bath is expendedless upon the evolution of hydrogen, and more upon the deposition of zinc.

Despite the advantages of cyanide-zinc baths,

they have not been much used by the art because of the poorappearance of zinc deposits obtainable therefrom. Regardless of the disadvantages in operation of acid-zinc baths, they have been favored by those working in the art because of the somewhat better appearing deposits obtain-1 It is an object of myinvention toprovide processes by means of which bright, smooth zinc elec-. trodeposits can be obtained.- .A further object of my invention is to provide processes by means of which the advantages of cyanide-zinc baths can be realized, and by means of which, at the same time, deposits of pleasing appearance can be is to provide electroplating baths which are characteriz'ed by good throwing power, and which will produce bright zinc deposits over a relatively exgroup VIII series 4 posit in a mildly 2 tended range of current densities. A still further object of my invention is to provide processes which produce zinc deposits that will respond to dilute oxidizing bright dips. Further objects of my invention will become apparent hereinafter. I accomplish my objects, briefly, by the use of cyanide-zinc baths which contain a thiourea and which may contain, in addition,

I of Mendeleieffs Periodic System. Considering the features of my invention in greater detail, it is first noted that a cyanidezinc bath must be employed if satisfactory resuits are to be obtained. Certain illustrative cyanide-zinc baths will be given hereinafter, but

it will be understood that the principles of my invention are applicable to any of the. cyanidezinc baths known to the art. v

I have found that the thioureas which are soluble in the bath exercise a most profound eifect upon the character of the electrodeposit, while urea seems to be without eifect. Thiourea, itself, is my preferred addition agent, and it operates to produce bright, smooth zinc deposits of a very pleasing'appearance. Other thioureas, such as phenylthiourea and diphenylthiourea-disulfonic acid can, of course,

group VIII series 4 of the Periodic System have little or no effect upon the character of a zinc electrodeposit when used alone, these metals seem to exercise some synergetic action upon the thioureas, and the joint effect of such metals and thioureas is superior to the'efiects attributable to the action of either. The metals of group VIII series 4, iron, cobalt'and nickel, may be added to the bath in substantial amount in the form of a bath soluble compound, such asiron ferrocyanide, cobalt sulfate, nickel sulfate, cobalt chloride, nickel chloride, cobalt oxide, and nickel As the baths which contain metal brighteners are used, the content of metal brightener decreases by reason of drag-out and other similar factors, and it is necessary to add such metals to the bath from time to time. Instead of adding the metals of group VIII series .4.-in the form of their soluble compounda'the bathmay be constantly replenished by using a zinc 'anode'which contains a suitable proportion of the metal brightener. I v

While the zinc deposits produced according to the processes of my invention are very bright and smooth, it may sometimes be desirable further to brighten them, and more particularly to renderthem passive to finger staining and to other stains which might result from handling. Some of the thioureas tend't'oproduce deposits which, while bright, neverthelesshave a some- .what yellowish hue. By bright dipping the deoxidizing solution, the deposit will be rendered brighter, any slight color will be removed and the deposit'will be rendered passive.

Zincdeposits produced according to the teachings of my invention are preferably bright dipped with an acidic hydrogen peroxide solution. Such a solution is preferably made by mixing hydrogen acidic hydrogen peroxide solution, otherbright dips, such as an acidic chromic acid solution or a dilute nitric acid solution, may be used.

a metal of be used instead of thiourea. I have also found that while. metals of the The following illustrative examples are given more fully to point out my invention.

Example I A cyanide-zinc bath containing my preferred addition agent, thioureaitself. was madeup as follows: V

By the use of this bath, a bright, smooth sine deposit was obtained. The bright current density range of the bath extended from about twenty to fifty amperes per square foot. While the deposit was white, smooth, and bright. it had a slightly yellowish hue. To obtain a perfectly clear and bright finish and to render the surface passive. the plated articles weretreated by them in an acidified hydrogen peroxide solution. The bright dip used was up withliydrogen peroxide and sulfuric acid, the ratio of 11:0: to H2304 being about 16 to l by weight. v

temple 1 As an example showing the use "ofa metal of group VIII series 4 in conjunction with a thio- V urea, a bath of the following composition is given: 7

pl I; but using another thiou'remvras made up as follows;

bGrtgd results were obtained the use of Example IV V The following is given as an additional example of the use of metals of group VIII series 4 in conjunction with thioureas: I

Zinc oxide (ZnO) .45 Sodium hydroxide (No.08) --....Y .38 Sodium cyanide;macmucmnn.4...; 11o Nickel sulfate (msotsmolgnr... .f 2 Phenylthiourea-- 18-.

It is noted that the nickeisulfate is equiva- I lent to about forty-five. one-hiuidredths'of one gram ofnickel per liter.

While thiourea is shown in about amount in the above example-the quantities used may be greatly varied. I generally prefer touse between about i and 20 grams per liter of thiourea, while, morespecifieally,about 7 to 15- grams per liter "lead to the results. Still more specifically, I prefer to use about ten grs'msfper su pumum liter, but this amount may be varied somewhat, depending upon the bath compositon and operating conditions.

The metals of group VIII series 4 of Mendele- 5 jeffs Periodic System are preferably used in a form which is soluble in the bath to which they are to be applied. A substantial amount of a soluble compound of one of these metals should be used, and I preferably employ such an amount as will introduce not substantially below five onehundredths of one gram of metal per liter. More specifically, I usually prefer to use about onetenth to one gram per liter of one of these metals. It is noted that the use of metal brighteners in conjection with thioureas is particularly advantageous when relatively concentrated cyanide zinc baths are employed.

It will be readily understood that the quantity of metal brightener in the bath may be adjusted so and held relatively constant by, as above suggested, employing as an anode a suitable alloy of zinc V and metal brightener. A few typical alloys were made up with zinc and cobalt as follows:

25 Per cent 1. Cobalt 0.0015 2. Cobalt 0.38 3. Cobalt 0.9 4. Cobalt 1.32

30 It will be noted that in Example I, sodium cyanide is used in about the known proportion for baths of this character. In the other examples, a considerably larger proportion of sodium cyanide is used. I have found that electrodeposits of im- 3 proved color are obtained with baths of this typeif no less than about ninety grams per liter of sodium cyanide is employed. The bath of Example I, with eighty grams per liter of sodium cyanide, as is above noted, produced deposits 40 which had a slightly yellowish hue. Generally, I

prefer to use between about ninety and onehundred and thirty grams per liter of sodium cyanide with baths having about the indicated amounts of zinc oxide and sodium hydroxide,

45 while, more specifically, I prefer to use about onehundred grams per liter, say ninety-five to onehundred and five grams per liter. It will, of course, be understood that the baths may be more dilute with a consequent reduction in the number 50 of grams per liter of the constituents.

My novel addition agents may be marketed as such, or, if 'preferred, they may be sold admixed with one or more of the constituents of the bath withwhich, they are to be used. They may, of course, be used, if desired, with other bath modifying agents. For instance, a small amount. of a mercury compound may be used with'my novel addition agents so that zinc may be more 5 readily plated upon cast iron.

While I have shown certain specific baths, and certain specific proportions of ingredients, it will be understood that I do not intend to be restricted thereby, the scope of my invention being set forth 'v 10 in the following claims:

I claim:

1. A cyanide-zinc electroplating composition containing a thiourea.

2. A cyanide-zinc electroplating composition 15 containing thiourea;

3. A cyanide-zinc electroplating composition containng a thiourea and a substantial amount of a soluble metal compound of a metal of group VIII series 4 of the Periodic System.

M 4. Acyanide zinc electroplating bath containing about one to twenty grams per liter of thiourea.

5. A cyanide-zinc electroplating bath containing seven to fifteen grams per liter of thiourea.

6. A cyanide-zinc electroplating bath containing about ten grams per liter of thiourea.

7. In a process of electrodepositing zinc, the step comprising depositing zinc from a cyanidezinc bath in the presence of a thiourea.

8. In a process of electrodepositing zinc, the step comprising depositing zinc from a cyanidezincbath in the presence of thiourea.

9. In a process of electrodepositing zinc, the step comprising depositing zinc from a cyanidezinc bath in the presence of a thiourea. and a substantial amount of a soluble metal compound of a metal of group VIII series 4 of the Periodic System.

10. In a process of electrodepositing zinc, the step comprising depositing zinc from a cyanidezinc bath in the presence of about one to twenty grams per liter of thiourea.

11. In a. process of electrodepositing zinc, the step comprising depositing zinc from a cyanidezinc bath in the presence of seven to fifteen grams per liter of thiourea.

12. In a process of electrodepositing zinc, the step comprising depositing zinc from a cyanidezinc bath in the presence of about ten grams per liter of thiourea.

RICHARD O. HULL. 

